This invention relates to multimode optical fibers and more particularly to multimode optical fibers wherein a plurality of dopants are radially graded throughout the core in order to minimize modal dispersion.
It has been recognized that optical waveguides, the cores of which have radially graded index profiles, exhibit significantly reduced pulse dispersion resulting from group velocity differences among modes. This dispersion reducing effect, which is discussed in the publication by D. Gloge et al., entitled "Multimode Theory of Graded-Core Fibers" published in the Nov. 1973 issue of the Bell System Technical Journal, pp. 1563-1578, employs a radially graded, continuous index profile from a maximum value on-axis to a lower value at the core-cladding interface. The index distribution in this type of waveguide is given by the equation EQU n(r)=n.sub.1 [1-2.DELTA.(r/a).sup..alpha. ].sup.1/2 for r.ltoreq.a (1)
where n.sub.1 is the on-axis refractive index, n.sub.2 is the refractive index of the fiber core at radius a, .DELTA.=(n.sub.1.sup.2 -n.sub.2.sup.2)/2n.sub.1.sup.2 a is the core radius, and .alpha. is a parameter between 1 and .infin..
It was initially thought that the parabolic profile wherein .alpha. is equal to 2 would provide an index gradient that would minimize dispersion caused by group velocity differences among the modes. Thereafter, other values of .alpha. were derived for the purpose of improving various optical properties such as lowering attenuation and providing high bandwidths over broad bands of wavelengths. For example, see U.S. Pat. Nos. 3,904,268; 4,006,962; and 4,057,320. The aforementioned Gloge et al. publication also pertains to an attempt to minimize dispersion by setting .alpha. equal to 2-2.alpha..
To make a high bandwidth multimode optical waveguide fiber it is necessary to precisely control the radial index of refraction of the core. A common method of forming such fibers is taught in U.S. Pat. Nos. 3,823,995 to Carpenter and 3,711,262 to Keck and Schultz, which patents teach examples of gradient index optical waveguides as well as examples of the formation of optical waveguides by inside vapor phase oxidation processes. Both of these patents are expressly incorporated herein by reference. The inside vapor phase oxidation processes include chemical vapor deposition, flame hydrolysis and any other processes by which vaporous material is directed into a heated tube, reacted with oxygen under the influence of heat and deposited on the inside wall surface of said tube. The material is deposited within the tube in successive layers and the tube is then removed from the heat to leave a fused blank. As will be understood, the central hole may be collapsed at the end of the deposition process, the blank may subsequently be reheated and the hole collapsed, or the hole may be collapsed during the drawing process. In any event, the blank or preform is subsequently heated and drawn into an elongated, fine strand. Inasmuch as the structure of the drawn strand or filament reflects the structure of the drawing blank or preform, it is important that the physical characteristics of the blank be carefully controlled.
In order to effect such change of the index of refraction of a blank or preform being formed by an inside vapor phase oxidation process, the chemical composition of the source materials, which, after reaction, comprise the ultimate material deposited on the inside surface of the tube, may be varied. The vapor mixture is hydrolyzed or oxidized and deposited on the inside surface of the tube and subsequently fused to form a high quality and purity glass. At the same time, one or more additional vapors can be supplied to the tube, each vapor being constituted of a chemical termed a "dopant" whose presence affects the index of refraction or other characteristics of the glass being formed.
In general, the method of forming optical waveguide blanks or preforms by the inside vapor phase oxidation process includes forming a barrier layer on the inside of the support or substrate tube prior to the deposition of the core glass with the substrate tube being the cladding. The principal function of the barrier layer is to minimize interface scattering and absorption losses by removing the core-cladding interface which would exist between deposited layers of high purity, low attenuation glasses and the substrate tube inner surface. The barrier layer is conventionally a borosilicate glass composition since doping silica, which is generally the base glass, with boron reduces the deposition temperature and thereby minimizes shrinkage of the substrate tube. Other advantages of doping silica with boron are that it reduces the refractive index of the glass and it acts as a barrier to the diffusion of hydroxyl ions, commonly referred to as water, from the substrate tube to the deposited core glass at the elevated processing and drawing temperatures.
It has been found that the bandwidth of an optical waveguide filament produced by the inside vapor phase oxidation process sometimes falls far short of the predicted theoretical value. For example, an attempt was made to form low loss high bandwidth optical waveguide filaments of SiO.sub.2 doped with B.sub.2 O.sub.3, GeO.sub.2 and P.sub.2 O.sub.5 in amounts represented by curves 10, 12 and 14 of FIG. 1. In this Figure, "CL." refers to cladding, "B.L." refers to barrier layer, and the radii r.sub.o, r.sub.a, r.sub.b and r.sub.c refer to the filament axis, the core radius, the barrier layer radius and the outside or cladding radius, respectively. As noted in U.S. patent application Ser. No. 929,416, "High Bandwidth Optical Waveguides and Methods of Fabrication" filed July 31, 1978, Olshansky et al., now U.S. Pat. No. 4,230,396, such a filament exhibits a combination of step-graded index of refraction profile which causes pulse spreading of higher order modes, a factor which lowers bandwidth. One of the causes of the step-graded profile is the abrupt elimination of B.sub.2 O.sub.3 at core-barrier layer interface. Because of the profile discontinuity at the edge of the core, the average bandwidth of this type of prior art optical filament is only about 240 MHz. Differential mode delay (DMD) analysis of this type of filament consistently revealed that the lower order modes were concurrent, but that the higher order modes were increasingly delayed, the highest order modes showing from 1 to 3 ns/km relative delay. More than half of the propagating modes showed relative delay.
The bandwidth values reported herein are as measured with a "mode scrambler" apparatus of the type described by W. F. Love in "Digest of Topical Meeting on Optical Fiber Communication" (Optical Society of America, Washington, D. C., 1979), paper ThG2, pp. 118-120. It is noted that these values may be considerably lower than those obtained without the use of a mode scrambler.
An attempt was made to improve bandwidth by eliminating the step-graded refractive index feature caused by the abrupt elimination of B.sub.2 O.sub.3 at the core-barrier layer interface. The B.sub.2 O.sub.3 concentration was graded from the barrier layer level to zero at a radius r.sub.d as shown by dashed line 16 while maintaining the concentrations of the other dopants at the values represented by curves 10, 12 and 14 of FIG. 1. In filaments wherein the radius r.sub.d was 27.2 .mu.m and the core radius r.sub.a was 31.25 .mu.m, the average bandwidth was 250 ; MHz at 900 nm. An examination of the differential mode delay of representative filaments showed that grading B.sub.2 O.sub.3 into the core is not without merit. FIG. 2, which is a DMD curve for a fiber wherein r.sub.d is 27.2 .mu.m shows that about 60% of the modes were relatively concurrent for that filament. This DMD curve indicates that a refractive index profile discontinuity is adversely affecting the higher order modes even when the abrupt step is eliminated by ramping the B.sub.2 O.sub.3 from the barrier layer level to some finite radius within the core.